1. Field
This invention pertains to devices and methods for performing optical measurements using a plurality of light spots, and more particularly, to a method of qualifying light spots for use for optical measurements by a measurement instrument, and a measurement instrument employing such a method of qualifying the light spots that are employed in its measurements.
2. Description
There are some devices which employ light spots to make optical measurements. One well-known example is the use of a Shack-Hartmann wavefront sensor.
FIG. 1 illustrates some principal elements of a basic configuration of a Shack-Hartmann wavefront sensor 100. Shack-Hartmann wavefront sensor 100 comprises a micro-optic lenslet array 110 and an optical detector 120. Typically, the optical detector 120 comprises a pixel array, for example, a charge-coupled device (CCD) camera or CMOS array.
The lenslets of the lenslet array 110 dissect an incoming wavefront and create a pattern of light spots 130 that fall onto optical detector 120. In one typical embodiment, lenslet array 110 includes hundreds or thousands of lenslets, each on the size scale of a hundred microns. Meanwhile, optical detector 120 typically comprises many pixels (e.g., 400 pixels) for each lenslet in lenslet array 110. Typically Shack-Hartmann sensor 100 is assembled such that the pixel array 120 lies in the focal plane of lenslet array 110.
Shack-Hartmann wavefront sensor 100 uses the fact that light travels in a straight line to measure the wavefront of light. By sensing the positions of light spots 130, the propagation vector of the sampled light can be calculated for each lenslet of lenslet array 110. The wavefront of the received light can be reconstructed from these vectors.
To better understand one or more aspects of this invention, it is worthwhile to discuss the operation of Shack-Hartmann wavefront sensor 100 in more detail. However, embodiments of the present invention extend to other types of optical measurement devices and systems such as topographers. In certain embodiments of the present invention, a system includes two or more optical measurement devices, for example, a combined system including both a wavefront sensor and a topographer.
In the case of the wavefront sensor 100, some optical system is employed to deliver a wavefront onto lenslet array 110, which samples the wavefront over the tiny regions of each lenslet. Beneficially, the lenslets are much smaller than the wavefront variation. For the purposes of this discussion, we define “isoplanatic” as the condition where the wavefront is well approximated by a plane wave over an area the size of a lenslet. In that case, the wavefront is preferably isoplanatic over the sampled region. When detector array 120 is in the focal plane of lenslet array 110, each lenslet will create a light spot on detector array 120. The location of these light spots reveals the average of the wavefront slopes across each region. That is, the shift in the location of a light spot is proportional to the average of the slope of the wavefront over the region sampled by the corresponding lenslet that produced the light spot. Software may compute the shift in each light spot.
In a typical operation, a reference beam (e.g., a plane wave) is first imaged onto lenslet array 110 and the locations of the resultant light spots (“reference locations”) on detector array 120 is recorded. Then, a wavefront of interest is imaged onto lenslet array 110, and the locations of the light spots on detector array 120 produced by the wavefront of interest is recorded and compared against the reference locations.
FIGS. 2A-F illustrate an idealized example of this process where a reference beam and a wavefront of interest are imaged onto a detector array of a wavefront sensor. This idealization shows the process of measuring a spherical wave with a wavefront sensor with just 16 lenslets. The first step, as represented by the FIGS. 2A-2C, is to measure a plane wave and measure the corresponding series of light spot locations 210 which are used as reference locations 220. The next step, as depicted in FIGS. 2D-2F, is to introduce a wavefront of interest and determine the shifts in the locations 240 of the light spots 230 from their reference locations 220.
If the wavefront is not isoplanatic, the quality of the light spot erodes rapidly and it becomes more difficult to determine the location. However, where the isoplanatic condition is satisfied and where the light spot shift is consistent with the small angle approximation of Fresnel, then the light spot shift is exactly proportional to the average of the wavefront slope over the lenslet. The incident wavefront is then reconstructed from the measurements of the average of the slopes for the hundreds or thousands of lenslets in the lenslet array.
Further details regarding the construction and operation of a Shack-Hartmann wavefront sensor and a system for measuring aberrations in an eye using the Shack-Hartman wavefront sensor are described in U.S. Pat. No. 7,122,774, issued on 17 Oct. 2006 to Daniel R. Neal et al., the entirety of which is hereby incorporated by reference for all purposes as if fully set forth herein.
One important application for Shack-Hartmann wavefront sensors is in the field of ophthalmic aberrometry. In common practice, a measurement instrument employing a Shack-Hartmann wavefront sensor injects near infrared light into a patient's eye which focuses on the retina and scatters back toward the instrument. This light is imaged onto the Shack-Hartmann lenslet array, and each lenslet in the lenslet array focuses the local portion of the incident light it intercepts onto the detector array, as described above. Data pertaining to the locations of the light spots is used to derive slope information using a least squares fit method, and thereby to construct the wavefront of the received light. The quality of the fit data, usually evaluated using Zernike coefficients, is affected by the quality of the light spot location data, and every effort is made to ensure the data quality is adequate to the measurement accuracy and precision requirements.
FIG. 3 shows a typical raw image from a wavefront sensor. The nominally rectilinear array of light spots is produced by a rectilinear lenslet array. The detailed analysis of the locations of these light spots relative to their reference locations (i.e., the locations that result when a true plane wave is applied to the lenslet array) yields the local gradient of the incident wavefront. The overall area in which focal spots are present is determined by the patient's pupil, and analysis of this illuminated area yields the location size and shape of the pupil.
The application of Shack-Hartmann wavefront sensors to ophthalmic aberrometry has been a success. However, improvements may be provided by eliminating or reducing the effects of errors that may be caused by complicating factors inherent to the measurement method. Some of the important error sources are illustrated in FIG. 3 and will be described below.
The incident near infrared beam not only scatters from a patient's retina, but also reflects directly from the patient's cornea. The use of a Range Limiting Aperture (RLA) in the measurement instrument, as described in U.S. Pat. No. 6,550,917 issued on 22 Apr. 2003 to Daniel R. Neal et al., can significantly reduce the intensity of the reflected light (U.S. Pat. No. 6,550,917 is hereby incorporated by reference in its entirety for all purposes as if fully set forth herein). However, this so-called “corneal reflex” is generally orders of magnitude brighter than the desired retinally scattered light, and—beneficially—may be excluded from the wavefront calculations. Indeed, as is illustrated by reference numeral 310 in FIG. 3, the reflex can affect a neighborhood of nearby focal spots by introducing stray light that can alter the true light spot location or mask the light spot entirely. The location and intensity of the corneal reflex is affected by corneal shape and the actual position of the patient's eye when the data is acquired. For these reasons, the qualification and/or exclusion of light spot data in and around the corneal reflex can be challenging.
The retinal scatter that is necessary for the aberrometer measurement is highly speckled because the retinal structure is quite rough compared to the wavelength of the probe beam. This leads to variability in the relative brightness of the focal spots. A measurement instrument may employ a broadband probe beam to reduce the speckle, but even so, the intensity of the light spots can vary by a factor of four in a normal clear eye. Additional variation can be introduced by cataracts, “floaters” and opaque regions in pathological crystalline lenses. Cataracts diffuse the incident and return beams causing both reduced spot intensity and broader light spots. As a result, as shown in FIG. 3, the raw image from the wavefront sensor may include dim light spots 320 and/or missing light spots 330.
Additional reflections of the probe beam may be produced by each surface in eyes implanted with intraocular lenses (IOLs). While similar to the corneal reflex phenomenon, multiple reflections are typically present in these patients and may be far from the optic axis of the wavefront sensor. Also, in subjects with diffractive IOLs, it is expected that one lenslet focal spot per diffraction order transmitted through the optical system may be present. In some cases this will lead to two or more focal spots that may or may not be spatially separated. Such focal spot distributions can lead to inaccurate spot location and therefore inaccurate wavefront measurements.
Another source of error in wavefront measurements is tear film breakup. Tear film break up can affect the location and sharpness of the light spots in the vicinity of the breakup. Tear film breakup is correlated to delays in blinking the eye. Some measurement systems may be designed to operate rapidly and reduce tear film breakup effects by avoiding the need to keep the patient from blinking for long periods. Nevertheless, it is still possible that light spots are affected by tear film breakup. This can negatively impact the resultant wavefront measurements.
The spot location algorithms used with a typical wavefront measurement instrument are designed to work with data taken within the nominal linear range of the detector device (e.g., a CMOS detector). Obviously the spot location information is compromised when the spot brightness is poor compared to stray light and camera noise. As described in U.S. Pat. No. 6,550,917, a wavefront measurement instrument may employ a dynamic range limiting aperture (RLA) to significantly enhance its immunity to stray light. However some environmental factors may lead to increased stray light levels; e.g., pointing the system toward a bright light source. A wavefront measurement instrument may incorporate high quality digital CMOS cameras to minimize the effects of camera dark noise. In that case, spots with many pixels that saturate the detector will yield less accurate spot location information.
Therefore, it would be desirable to provide one or more methods of qualifying which light spots are used for optical measurements by a measurement instrument. It would also be desirable to provide a measurement instrument employing a method of qualifying the light spots that are employed in its measurements.
In one aspect of the invention, a method employs an optical sensor to determine a property of an object. The method comprises: (a) illuminating the object with light from one or more light sources; (b) receiving light from the illuminated object; (c) producing a group of light spots from the received light; (d) qualifying a set of the light spots for use in determining a property of the object; and (e) determining the property of the object using the qualified set of light spots. Qualifying the set of light spots includes, for each light spot in the group of light spots: calculating a first calculated location of the light spot using a first calculation algorithm; calculating a second calculated location of the light spot using a second calculation algorithm different from the first calculation algorithm; and when a difference between the first and second calculated locations for the light spot is greater than an agreement threshold, excluding the light spot from the qualified set of light spots. In addition, the light spots excluded from the qualified set may be excluded from being employed in determining the property of the object. Alternatively, one or more of the spots excluded for the qualified set of light spots may be considered for inclusion in a second set of light spots. Some or all of the second set of light spots may also be used in determining the property of the object, for example, by assigning a lower weighting than those spots in the qualified set. Alternately, some or all of the second set of light spots may be used to detect, measure, or characterize some feature of the optical system or eye, e.g., cataracts, tear film conditions, surface anomaly, or the like.
In another aspect of the invention, a device includes: one or more light sources for illuminating an object; a light spot generator adapted to receive light from the illuminated objected and to generate a group of light spots from the light received from the illuminated object; a detector adapted to detect the light spots and for outputting light spot data pertaining to each light spot; and a processor adapted to process the light spot data to determine a property of the object. The processor processes the light spot data by: qualifying a set of the light spots for use in determining the property, and determining the property of the object using the qualified set of light spots. Qualifying the set of light spots includes, for each light spot in the group of light spots: calculating a first calculated location of the light spot from the light spot data using a first calculation algorithm; calculating a second calculated location of the light spot from the light spot data using a second calculation algorithm different from the first calculation algorithm; and when a difference between the first and second calculated locations for the light spot is greater than an agreement threshold, excluding the light spot from the qualified set of light spots. Beneficially, in addition, the light spots excluded from the qualified set of light spots may be excluded from being employed in determining the property of the object.
In yet another aspect of the invention, a method comprises: producing a first set of first light spots from an eye with a corneal topography measurement; producing a second set of second light spots from the eye with a wavefront aberrometry measurement; and
qualifying one or more of the light spots within one of the first and second set of light spots based on the other of the first and second set of light spots.
In still another aspect of the invention, a method is provided for determining a condition of an eye. The method comprises: providing a wavefront aberrometer with a first light source and a topographer with a second light source; illuminating an eye with the first light source to produce a first group of light spots; receiving the first group of light spots at a first detector array to produce a first signal containing a first set of data; illuminating the eye with the second light source to produce a second group of light spots; receiving the second group of light spots at a second detector array to produce a second signal containing a second set of data; comparing the first set of data to the second set of data; and based on the comparison, determining an abnormality of the eye.